Nickel electrodes for alkaline batteries fall primarily into one of two categories, i.e. sintered or nonsintered. Sintered electrodes typically employ porous, sintered nickel plaques electrochemically impregnated with nickel hydroxide and have demonstrated excellent peak power retention after repeated cycling. Nonsintered nickel electrodes (often called plastic-bonded electrodes) are made by pressing a mass of active material onto a conductive substrate (e.g., expanded metal, wire mesh, foils, etc., made from copper, nickel, iron, etc.) and, compared to sintered electrodes, have generally poor peak power retention after cycling. In nonsintered, plastic-bonded electrodes, the active material mixture typically comprises: nickel hydroxide as the electrochemically active material; conductive diluent (e.g., nickel or graphite particles) distributed throughout the nickel hydroxide for imparting conductivity to the mass; cobalt hydroxide to improve the charge efficiency of the electrode; and a plastic binder (e.g., styrene, PTFE, etc.) for holding the active material and diluent together onto the conductive substrate. More specifically, these active material masses optimally contain about 20% to about 25% percent by weight of graphite as the conductive diluent in particles which range in size up to about ten microns. Lower conductive diluent loadings generally tend to reduce the conductivity and power performance of the active material while greater diluent loadings unnecessarily consume space otherwise available for the active nickel hydroxide without any significant offsetting benefit. The binder content of the active mass can vary from about 1 to about 10 percent by weight of the active mass depending on the particular binder chosen. A PTFE binder content, for example, of about three to about five percent by weight is not unusual. Most manufacturers also include a small amount of cobalt hydroxide (i.e., equal to about five percent of the nickel present) to improve charging of the electrode.
The active mass of the nonsintered electrodes is pressed (e.g., by rolling) onto a metallic substrate (e.g., nickel or nickel plated copper) which serves not only to support the active material relative to the battery's counterelectrode but also as the primary current collector for the electrode. This conductive substrate can take many forms such as expanded metal, stabbed foil (i.e., a punctured sheet with puncture tears projecting from both sides thereof), open-cell foam metal, etc.
Nonsintered plastic-bonded nickel electrodes are more economical to produce than their sintered counterparts and offer the distinct advantage of utilizing manufacturing techniques and equipment quite similar to those commonly used for pasting Pb-acid battery plates. However, nonsintered electrodes have not achieved the same power performance levels as the sintered electrodes. In this regard, nonsintered nickel electrodes typically have a lower initial peak power than sintered electrodes and, unlike sintered electrodes, their peak power (e.g. at 50% depth of discharge-DOD) fades quite rapidly upon repeated cycling. One of the major causes of this power fade in nonsintered nickel electrodes is believed to be the interfacial resistance that exists between the active material mass and the substrate which resistance increases dramatically with repeated cycling. Untoward interfacial resistance increase upon repeated cycling is believed to be associated with: low interfacial contact between the active material and the surface of the substrate; loss of adhesion between the active material and the substrate; and the formation of an resistive oxide film on the surface of the substrate.
Heretofore, others have observed that the performance of nonsintered electrodes improves as the roughness of the substrate's surface increases. For example, V. Koudelka et. al., Plastic-Bonded Electrodes for Nickel-Cadmium Accumulators. V. Influence of the Current Collector and Mechanical Compression on the Current Carrving Capability of the Nickel Oxide Electrode, Journal of Power Sources, 6 (1981), 161-169 noted that a "metallized" (e.g., sprayed) nickel surface was superior to an electroplated nickel surface. Moreover, others have provided roughened substrate surfaces by electrolytically codepositing nickel and graphite onto the substrate. These latter coatings, sometimes referred to as "codep" coatings, have proved more effective than other earlier used electrodeposits (e.g., bright, semibright, or mat nickel deposits) in improving the power performance of nonsintered nickel electrodes. Codep coatings however are quite expensive to produce, difficult to control on a production basis and have a roughness which is essentially dependent on the particle size of the conductive inclusions.
Accordingly, it is an object of the present invention to provide a nonsintered nickel electrode having a power performance at least substantially comparable to electrodes made with codep coated substrates but which can be manufactured more readily, economically and consistently, and whose properties can be varied simply by varying easily controllable plating parameters such as time, temperature, concentration and current density. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.